Is your file sorted by the internal ID/however you are telling NS which records to update?

Yes, that is the scenario, I am mapped to the record on External ID and the file is sorted by External ID. Would this be the source of the error?

@Alex K if you have the same item listed multiple times on different lines of a Sales Order you cannot update it via CSV import. You either need to check the overwrite sublist a box in the advanced options of the csv import OR Edit the SO, click the clear all lines button, add 1 placeholder line, then import to a blank SO

Not entirely true. You can map a line id